The present invention relates to pistons and particularly to pistons for internal combustion engines.
Broadly speaking a piston may be categorised into two regions; the crown and piston ring belt forming one region and the skirt including the gudgeon pin bosses forming the second region.
When being machined to final shape the crown and ring belt region is usually machined to a circular polar profile. There are, however, exceptions; specific designs of some engines having deviations from this generalisation. The skirt region, however, is generally neither circular in polar profile nor linear in axial profile.
The skirt region is often machined on a cam turning machine to an oval polar profile. One example of such an oval polar profile is an ellipse where the minor diameter is parallel to the gudgeon pin axis and the major diameter lies in a direction at right-angles to the gudgeon pin axis. The ovality of a piston may be defined as the difference between the dimension of the major diameter and that of the minor diameter. The resulting figure of ovality, however, only gives the maximum deviation from a true circular polar profile and gives no indication of the shape of the curve between the major and minor diameters. Any given ovality figure could in theory be applied to an infinite number of different curves.
The reasons for machining piston skirts to shapes other than circular in polar profile include some based on thermal considerations and others based on dynamic considerations relating to the environment in which the piston operates.
Since for practical reasons pistons may only be economically produced by machining at ambient temperatures the form to which a piston is machined is an attempt to compensate for the thermal distortion which occurs during heat-up to and dynamic operation at the normal running temperature of the piston. Such distortion may be anything but uniform and will be greatly influenced by the temperature variations which occur within the piston itself. Temperatures will be higher nearer to the crown and lower at the lower skirt portions. Furthermore, the changes of section thickness in the skirt region will also influence the manner and degree of the distortion and will vary from the thin sections of the skirt per se to the thicker, stiffer regions around the pin boss and ring belt.
The dynamic considerations to be made stem from the stress/strain effects during operation of the engine. The forces acting on the piston are generally at a mximum during the power stroke of the engine during combustion of the fuel. The forces due to combustion are borne mainly on one side of the piston known as the thrust face of the piston. The forces generated in the remainder of the cycle are much lower and are borne both by the thrust face and the other side of the piston which is known as the counter-thrust face. These forces are not necessarily, however, spread evenly between the thrust and counter-thrust faces of the piston.
The curvature to which the piston is machined generally attempts to produce in the running engine a "bedding" or contact area between the piston and its associated cylinder wall which lies within an arc subtending between approximately 40.degree. and 80.degree. on the thrust and counter-thrust faces of the piston.
Heretofore piston skirts have been machined symmetrically about the plane which includes both the piston and the minor axis. The curvature of the piston skirt has generally been calculated to produce the desired bedding area to accommodate the combustion generated forces on the thrust face of the piston. It has now been discovered that significant improvements in, for example, noise reduction, stability and scuffing between a piston and its associated cylinder wall may be obtained by machining non-symmetrically about the plane which includes both the piston axis and the minor axis.